1. Field of the Invention
This invention relates to an electronic endoscope for displaying a body-cavity image on a TV monitor, the endoscope having a video-scope with an image sensor and a video-processor. In particular, the present invention relates to adjusting the brightness of an object image.
2. Description of the Related Art
In an electronic endoscope, a video-scope includes an optical light guide extended therethrough, which is formed as a bundle of optical fibers. A video-processor includes a light source, such as a halogen lamp. When the video-scope is connected to the video-processor, a proximal end of the optical light guide is optically connected to the light source. Thus, an object to be photographed is illuminated by light radiating from a distal end of the optical light guide, and an object image is formed on the image sensor provided at the distal end of the video-scope. Then,the object image,formed on the image sensor, is converted into analog image-pixel signals by photoelectric conversion.
The analog image-pixel signals are fed to the video-processor and are suitably processed, so that video signals are generated. The video signals are then output to a TV monitor, thus a body-cavity image (for example, a stomach image) is displayed on the monitor.
In general, to maintain proper brightness of the object image displayed on the monitor, a quantity of light radiating from the distal end of the optical light guide should be automatically regulated. In a conventional endoscope, for regulation of the quantity of light, a stop (diaphragm) provided between the light source and the proximal end of the light guide is controlled, such that the brightness of the object image displayed on the monitor is maintained at a constant level. The luminance value, indicating the brightness of the object image, is successively calculated on the basis of the image-pixel signals, and then the control of the stop is performed at regular time-intervals. In controlling the stop, the drive-amount of the stop is determined on the basis of the difference between the luminance value and a reference value. Note, the luminance value is, for example, an average value of one frame worth, and the reference value is, for example, a middle value of a range of luminance values (from 0 to 255), which corresponds to the proper brightness of the object image.
However, in the conventional automatic adjustment of the quantity of light, a drive time of the stop becomes very long when the determined drive-amount of the stop is large. The luminance value is successively calculated independently of the drive-amount of the stop. Therefore, the stop is occasionally driven on the basis of the preceding luminance value.
When the stop is driven in error, the brightness of the object image formed on the image sensor having already been corrected, hunting, in which the stop does not converge to a proper position for a long time, occurs, so that the brightness of the object image can not be quickly adjusted properly.
On the other hand, if the regulation of the quantity of light is performed at longer time-intervals in order to solve the above problems, the stop cannot be rapidly controlled when the brightness of the object image changes considerably.
Therefore, an object of the present invention is to provide an electronic endoscope that can properly adjust brightness of an object image by controlling the stop, without causing hunting.
An electronic endoscope of the present invention includes a video-scope, video-processor, a light source, a luminance value calculator, a stop, a motor, a drive-amount setter, a stop-controller, and a drive-amount determiner. The video-scope has an image sensor, provided at a distal end of the video-scope, on which an object image is formed, and a light guide provided throughout the video-scope, to guide light from a proximal end of the video-scope to the distal end of the video-scope. The video-processor, to which the proximal end of the video-scope and a monitor for displaying the object image are respectively connected, processes image-pixel signals corresponding to the object image, successively read from the image sensor. The light source is provided in the video-processor such that light, emitted from the light source, is guided through the light guide and radiates from the distal end of the video-scope. The luminance value calculator successively calculates a luminance value indicating brightness of the object image, on the basis of the image-pixel signals. The stop is provided between the light source and the proximal end of the video-scope, and regulates a quantity of light radiating from the distal end of the video-scope. The motor drives the stop, the stop being connected to the motor such that the stop is driven in accordance with a motion of the motor. The drive-amount setter sets a target drive-amount of the stop, to make the brightness of the object image constant, on the basis of the luminance value. The stop-controller adjusts the quantity of light radiating from the distal end of the video-scope by controlling the stop in accordance with the target drive-amount at regular time-intervals, whereby the stop opens and closes such that the brightness of the object image displayed on the monitor is maintained at constant level. Then, the drive-amount determiner determines whether the target drive-amount exceeds a tolerable drive-amount. When the target drive-amount exceeds a tolerable drive-amount, the stop-controller at least suspends driving of the stop at the next adjustment of the quantity of light.